Aircraft gas turbine engines include various rotors having blades rotating therewith such as fan, compressor, and turbine blades which generate vibratory excitation forces during operation. Accordingly, the engine is conventionally designed to maximize the margin between the various operational excitation frequencies and the critical frequencies associated with the natural resonance modes of vibration to minimize undesirable vibration and for obtaining a suitable useful life of the engine during operation.
The vibratory response of the engine is typically affected by its supporting structure in the aircraft. For example, an aircraft engine is typically supported to an aircraft wing by a conventional pylon. The aircraft wing has inherent flexibility in the vertical direction for example, which therefore provides a flexible support of the engine mounted thereto. In the past, the installed vibratory response of the engine was affected little by the flexible wing mounting, and therefore engine design and development assuming a relatively rigid support provided acceptable results.
For example, during the development of an aircraft gas turbine engine the various stator and rotor components thereof must be suitably designed for obtaining suitably low vibration during operation. In order to be certified for an aircraft application, the engine is suitably tested for vibratory response to ensure acceptable vibratory levels. Upon completion of the certification program and during production of the engines, each engine is typically tested before shipment in a conventional stationary test stand as well as being typically flight tested in an aircraft to ensure acceptably small levels of vibration. If vibration is excessive, the engine requires rebalancing to meet the appropriate specifications which involves additional procedures and time and is therefore relatively expensive to accomplish.
A typical test stand includes a vertical support column suitably mounted to a foundation in the ground, with a head frame at the top thereof from which individual aircraft engines are removably mounted for undergoing testing. During ground testing, the engine is operated at various levels of output power thrust and various components thereof are monitored for vibration levels. The test stand itself is typically a rigid structure having a spring constant of an order of magnitude greater than that of the stiffness of the pylon supporting the engine to the test stand. Conventional test stands typically include a removable adaptor which mounts the engine and pylon to the test stand head frame for allowing ready assembly and disassembly of the engine to the test stand. A conventional adaptor is also a rigid component typically having a flexibility or spring rate an order of magnitude greater than that of the pylon and engine support.
Improvements in aircraft and engine design require lower levels of acceptable engine vibration and, therefore, improved test stands are desired for better testing the installed system dynamics of the engine as it would operate in an actual aircraft engine environment.